Polishing apparatus

ABSTRACT

A polishing apparatus comprises a polishing table having a polishing surface and a top ring for holding a substrate to be polished, in which the substrate held by the top ring is pressed against the polishing surface of the polishing table and thus polished. A capacitance type sensor and/or an eddy-current type sensor is disposed in one or more location(s) in the vicinity of the top ring. The capacitance type sensor detects the escaping of the substrate to be polished based on a change in capacitance between the capacitance type sensor and a top surface of the polishing table. The eddy-current type sensor detects the escaping of the substrate to be polished based on a change in electrical resistance between the eddy-current type sensor and the top surface of the polishing table.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a polishing apparatus for polishing a workpiece to be polished, such as a semiconductor wafer and so on, and in more specific, to a polishing apparatus having a function of detecting an escaping/slipping-off of a workpiece to be polished from a workpiece holding mechanism during polishing.

[0002] In recent years, as a level of micro-miniaturization and high-integration of semiconductor device progresses, a space between wirings is getting much narrowed. Especially, in a photo lithography with a line width equal to or less than 0.5 μm, a shallow focal depth thereof requires a high level of flatness in a plane on which an exposing apparatus forms an image. To realize such high level of flatness, a polishing apparatus has been broadly employed to provide an effective polishing operation.

[0003] This type of polishing apparatus typically comprises a polishing turntable with a polishing cloth affixed on a top surface thereof and a top ring body. The turntable and the top ring body are driven to rotate at their independently determined revolving speed. A substrate to be polished, which has been held in the top ring body, is pressed against the polishing surface of the turntable. A surface of the substrate to be polished is polished into a flat and mirror-finished surface while supplying an abrasive liquid or slurry to the polishing surface. After the polishing operation having been finished, the substrate to be polished is removed from the top ring body and transferred to a subsequent process, a cleaning process for example.

[0004] The polishing apparatus, however, has been suffered from a problem that the substrate to be polished is occasionally cracked during polishing and fragments of the broke substrate are dispersed across the polishing cloth. If such a polishing cloth having the fragments of the broke substrate to be polished remaining dispersed across the polishing cloth is reused, the polishing cloth could make scratches on the surface of the substrate to be polished. Due to this, the polishing cloth has to be replaced with another each time when the substrate to be polished is cracked.

[0005] There has been still another problem, even in the case of no cracking of the substrate to be polished, that the substrate to be polished occasionally slips out of the top ring body. In this case, if the substrate to be polished is composed of fragile material as represented by a silicon wafer and so on, the substrate could impinge upon a wall surface of a casing covering the turntable and occasionally develop a damage, such as a chipping, in a peripheral region of the substrate to be polished. If this damaged substrate is to be polished again, only a light load applied to the vicinity of the damaged area could crack the substrate.

[0006] To address the above-pointed problems, a polishing apparatus as disclosed in Japanese Patent Laid-open Publication No. 2001-96455 comprises a sensor of ultrasonic type, which is disposed on an outside of the top ring to measure a distance to the top surface of the turntable. When the distance to the top surface of the turntable, which is measured by the sensor of ultrasonic type, is changed due to the intervening of the substrate to be polished that happens to slip out onto the surface of the turntable to be measured, the sensor detects this as an abnormal polishing or a slipping-off of the substrate to be polished.

[0007] Further, another polishing apparatus as disclosed in Japanese Patent Laid-open Publication No. 2001-96455 includes a condenser comprising electrode plates disposed to sandwich the substrate to be polished, which has been held by the top ring, from both sides thereof. Alternatively, the polishing apparatus may include a condenser comprising electrode plates disposed in locations to sandwich the substrate to be polished which happens to slip out of the top ring. A constant voltage is applied to such a condenser, so that the abnormal polishing or the slipping-off of the substrate to be polished may be detected based on a current flowing through the condenser.

[0008] Still further, another polishing apparatus as disclosed in Japanese Patent Laid-open Publication No. 2001-96455 includes a contact element to be in contact with the turntable in a location defined on an under surface or a periphery of the top ring. A current is applied between this contact element and the turntable surface, so that the abnormal polishing or the slipping-off of the substrate to be polished may be detected based on a change in the current flowing therethrough.

[0009] However, either one of the above-described detection methods for determining the abnormal polishing or the slipping-off of the substrate to be polished has been also suffered from a problem that it is susceptible to the effect of noise and so on and thus not reliable. In addition, any one of those methods has been associated with another problem that it takes a long time to execute a signal processing and to finally determine whether or not the processed signal indicates the abnormal polishing or the slipping-off of the substrate to be polished, thereby disadvantageously leading to a state as allowing the substrate to be polished to impinge against, for example, the wall surface of the casing surrounding the turntable during waiting for an appropriate action, such as stopping of the top ring and/or the turntable, to be taken.

SUMMERY OF THE INVENTION

[0010] The present invention has been made in the light of the problems described above, and an object thereof is to provide a polishing apparatus capable of detecting an escaping or a slipping-off of a workpiece to be polished from a workpiece holding mechanism in a short time and thus allowing for any appropriate action to be taken quickly.

[0011] According to an aspect of the present invention, there is provided a polishing apparatus comprising:

[0012] a polishing table having a polishing surface;

[0013] a workpiece holding mechanism for holding a workpiece to be polished, the workpiece held by the workpiece holding mechanism being pressed against the polishing surface of the polishing table so that the workpiece is polished through a relative motion of the workpiece to the polishing surface of the polishing table; and

[0014] at least one of a capacitance type sensor, an eddy-current type sensor, and a combination of the capacitance type sensor with the eddy-current type sensor being disposed in at least one location in a vicinity of a workpiece holding section of the workpiece holding mechanism,

[0015] the capacitance type sensor detecting an escaping of the workpiece to be polished based on a change in capacitance between the capacitance type sensor and a surface of the polishing table,

[0016] the eddy-current type sensor detecting the escaping of the workpiece to be polished based on a change in electrical resistance between the eddy-current type sensor and the surface of the polishing table.

[0017] As described above, by employing such a configuration in which either one of the capacitance type sensor for detecting the escaping of the workpiece to be polished based on the change in the capacitance between the capacitance type sensor and the surface of the polishing table, the eddy-current type sensor for detecting the escaping of the workpiece to be polished based on the change in the electrical resistance between the eddy-current type sensor and the surface of the polishing table, or a combination of these two sensors is disposed in one or more location(s) in the vicinity of the workpiece holding section, the escaping/slipping-off of the workpiece to be polished from the workpiece holding mechanism can be detected quickly (in a short time), as will be described later in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIGS. 1(a) and 1(b) is schematic diagrams showing an exemplary general configuration of a polishing apparatus according to one embodiment of the present invention, wherein FIG. 1(a) is a plan view showing an arrangement of a top ring and a polishing table, while FIG. 1(b) is a side elevational view of the polishing apparatus;

[0019] FIGS. 2(a) and 2(b) are schematic diagrams for illustrating a capacitance type sensor of the polishing apparatus according to one embodiment of the present invention, wherein FIG. 2(a) is a schematic view showing a state of a substrate to be polished, which is held normally on a lower surface of the top ring, while FIG. 2(b) is a schematic view showing a state of the substrate to be polished, which has partially escaped/slipped out of the lower surface of the top ring;

[0020]FIG. 3 is a schematic diagram showing a general configuration of a capacitance measuring system of the polishing apparatus according to one embodiment of the present invention;

[0021]FIG. 4 is a block diagram showing an exemplary circuit design for detecting a change in capacitance by using the capacitance type sensor;

[0022] FIGS. 5(a) and 5(b) are schematic diagrams for illustrating an eddy-current type sensor of a polishing apparatus according to another embodiment of the present invention, wherein FIG. 5(a) is a schematic view showing a state of a substrate to be polished, which is held normally on a lower surface of a top sing, while FIG. 5(b) is a schematic view showing a state of the substrate to be polished, which has partially escaped/slipped out of the lower surface of the top ring;

[0023]FIG. 6 is a block diagram showing an exemplary circuit design for detecting a resistance between a lower surface of the eddy-current type sensor and a top surface of a polishing table by using the eddy-current type sensor;

[0024]FIG. 7 is a schematic diagram showing an arrangement of a sensor electrode of a polishing apparatus according to still another embodiment of the present invention; and

[0025]FIG. 8 is a schematic perspective view showing a general configuration of a polishing apparatus according to yet still another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0026] One embodiment of the present invention will now be described with reference to the attached drawings. FIGS. 1(a) and 1(b) are schematic diagrams showing an exemplary general configuration of a polishing apparatus according to one embodiment of the present invention. FIG. 1(a) is a plan view showing an arrangement of a top ring and a polishing table. FIG. 1(b) is a side elevational view of the polishing apparatus.

[0027] Referring to FIGS. 1(a) and 1(b), reference numeral 1 designates a polishing table (i.e., a turntable). A polishing cloth (a polishing pad) 2 is affixed onto a top surface of the polishing table 1. The polishing table 1 is supported by a revolving shaft 3. The polishing table 1 and the polishing cloth 2 are designed to be driven by the revolving shaft 3 so as to rotate in a direction indicated by an arrow A. Reference numeral 4 designates a top ring (i.e., a workpiece holding mechanism). The top ring 4 holds a substrate to be polished, W, such as a semiconductor wafer and so on, on its lower surface. The top ring 4 is mounted to a lower end of a top ring revolving shaft 5. The top ring revolving shaft 5 is operatively supported by a top ring swing arm 6 so as to rotate in a direction indicated by an arrow B. Further, the top ring 4 is adapted to move up and down along with the top ring revolving shaft 5 with respect to a polishing surface of the polishing table 1, namely the polishing cloth 2, by an elevator means, though not shown. A lowering operation of the elevator means moves down the top ring 4 to thereby cause the substrate to be polished, W, to be pressed against the polishing cloth 2 with a predetermined pressure applied thereon. Further, a lifting operation of the elevator means can move up the top ring 4 apart from the polishing cloth 2.

[0028] The top ring swing arm 6 is fixed to a swing shaft 7 and adapted to swing (rotate) by the swing shaft 7 in a direction indicated by an arrow C. Reference numerals 8, 9 and 10 individually designate sensors for detecting the escaping/slipping-off of the substrate to be polished, W, which are disposed in the periphery of a workpiece holding section of the top ring 4. Those sensors 8, 9 and 10 are mounted to the swing arm 6, respectively. Reference numeral 11 designates an abrasive liquid supply nozzle for supplying an abrasive liquid 12, such as a slurry, to the top surface of the polishing cloth 2 of the polishing table 1.

[0029] In the polishing apparatus having the configuration describe above, the substrate to be polished, W, which is held on the lower end surface of the top ring 4 rotating about the top ring revolving shaft 5, is pressed against the top surface (i.e., the polishing surface) of the polishing cloth 2 of the polishing table 1 rotating about the revolving shaft 3 and thus polished while supplying the abrasive liquid 12 from the abrasive liquid supply nozzle 11. During this polishing operation, if the substrate to be polished, W, escapes/slips out of the top ring 4, the substrate to be polished, W, would intrude into a space between the lower end of at least one of the sensors 8, 9 and 10 and the surface of the polishing cloth 2. In this event, the at least one of the sensors 8, 9 and 10 can detect it.

[0030] Since the sensors 8, 9 and 10 serving for detecting the substrate to be polished, W, have been disposed in three locations in the periphery of the workpiece holding section of the top ring 4, either one of the sensors 8, 9 and 10 can detect the substrate to be polished, W, when it happens to escape/slip out of the workpiece holding section of the top ring 4 in any directions. In such a case where the direction of the possible escaping/slipping-off of the substrate to be polished, W, may be estimated with high reliability to be a certain direction, for example, the downstream direction with respect to the top ring viewed from the rotational direction of the polishing table, then the sensor(s) may be disposed in one or more location(s) in the periphery along the direction.

[0031] The sensors 8, 9 and 10 may be capacitance type sensors. Alternatively, they may be eddy-current type sensors. Further alternatively, the sensors 8, 9 and 10 may be such sensors as fabricated by combining the capacitance type sensor with the eddy-current type sensor. This means that at least one of the sensors (e.g., the sensor 8) may be configured by combining the capacitance type sensor with the eddy-current type sensor, or one of the sensors (e.g., the sensor 8) may be configured with the capacitance type sensor while the other sensors (e.g., the sensors 9 and 10) may be configured with the eddy-current type sensors.

[0032] FIGS. 2(a) and 2(b) are schematic diagrams for illustrating an embodiment, in which each of the sensors 8, 9 and 10 has been configured as the capacitance type sensor, respectively. FIG. 2(a) shows a state of the substrate to be polished, W, which is held normally on the lower surface of the top ring 4. FIG. 2(b) shows a state of the substrate to be polished, W, which has partially escaped/slipped out of the lower surface of the top ring 4.

[0033] In the state where the substrate to be polished, W, is normally held on the lower surface of the top ring 4 as shown in FIG. 2(a), for example, the capacitance type sensor 8 measures a capacitance generated by a gap G1 between the lower surface of the sensor 8 and the top surface of the polishing cloth 2 and the thickness G2 of the polishing cloth 2. This measured value is expressed as a reference value Cr.

[0034] On the other hand, when the substrate to be polished, W, has partially escaped/slipped out of the lower surface of the top ring 4 and intruded into the gap G1 between the lower surface of the sensor 8 and the top surface of the polishing cloth 2 as shown in FIG. 2(b), the capacitance type sensor 8 measures a resultant capacitance Cc generated by the gap G1 between the lower surface of the sensor 8 and the top surface of the polishing cloth 2 and the thickness G2 of the polishing cloth 2.

[0035] A change ΔC in capacitance between the lower surface of the sensor 8 and the top surface of the polishing table 1 is defined by an expression, ΔC=Cc−Cr, allowing the escaping/slipping-off of the substrate to be polished, W, to be detected quickly (in a short time).

[0036]FIG. 3 is a schematic diagram showing a general configuration of a capacitance measuring system for the case of each of the sensors 8, 9 and 10 configured as the capacitance type sensor.

[0037] Herein, an effective area of the sensor 8 is assumed to be A, the gap between the lower surface of the sensor 8 and the top surface of the polishing table 1 to be G1+G2, and the dielectric constant of a substance existing between the lower surface of the sensor 8 and the top surface of the polishing table 1 to be ε. Since the gap between the lower surface of the sensor 8 and the top surface of the polishing table 1 yields G1+G2, the capacitance between the lower surface of the sensor 8 and the polishing table 1 may be represented as:

C=(A×ε)/(G1+G2)

[0038] This means that the capacitance C may be changed when the substrate to be polished, W, having a different dielectric constant ε from that of the air intervenes between the lower surface of the sensor 8 and the polishing table 1. The control section 13 detects the change in this capacitance C and thus detects the escaping of the substrate to be polished, W.

[0039]FIG. 4 is a block diagram for illustrating an exemplary circuit design for detecting a change in capacitance by using the capacitance type sensor 8.

[0040] The circuit for detecting the change in capacitance comprises a detecting section 20. The detecting section 20 comprises an oscillator (e.g., a crystal oscillator) 21, a resistor 22, an amplifier 23, a commutating and smoothing device 24, an A/D converter 25 and a capacitance value converter 26.

[0041] An oscillation signal (a high-frequency signal) from the oscillator 21 is supplied to the sensor 8, while a signal from the polishing table 1 is amplified by the amplifier 23 and converted by the commutating and smoothing device 24 into a direct current signal, which is further converted by the A/D converter 25 into a digital signal, and thus obtained digital signal is in turn converted by the capacitance value converter 26 into an output voltage Vc which corresponds to a capacitance value. If the capacitance C between the lower surface of the sensor 8 and the polishing table 1 is changed, the signal current flowing from the sensor 8 to the polishing table 1 and thus the output voltage Vc are changed. Based on any change in this output voltage, the escaping/slipping-off of the substrate to be polished, W, from the top ring 4 can be detected.

[0042] FIGS. 5(a) and 5(b) are schematic diagrams for illustrating another embodiment, in which each of the sensors 8, 9 and 10 has been configured as the eddy-current type sensor, respectively. FIG. 5(a) shows a state of the substrate to be polished, W, which is held normally on the lower surface of the top ring 4. FIG. 5(b) shows a state of the substrate to be polished, W, which has partially escaped from the lower surface of the top ring 4.

[0043] In the state where the substrate to be polished, W, is normally held on the lower surface of the top ring 4 as shown in FIG. 5(a), for example, the eddy-current type sensor 8 measures an electrical resistance of the sensor 8 (i.e., the electrical resistance between the lower surface of the sensor and the polishing table). This measured value is expressed as a reference value Rr.

[0044] On the other hand, when the substrate to be polished, W, has partially escaped from the lower surface of the top ring 4 and intruded into a space between the lower surface of the sensor 8 and the top surface of the polishing cloth 2 as shown in FIG. 5(b), the eddy-current type sensor 8 measures the electrical resistance of the sensor 8. The measured value is represented by Rs. This measured value Rs and the reference value Rr are compared to each other to acquire the difference, ΔR=Rr−Rs, and thereby the escaping of the substrate to be polished, W, can be detected quickly.

[0045]FIG. 6 is a block diagram for illustrating an exemplary circuit design for detecting a resistance value between the lower surface of the eddy-current type sensor 8 and the top surface of the polishing table by using the eddy-current type sensor 8.

[0046] The eddy-current type sensor 8 has a sensor coil 41. The sensor coil 41 is made up of an air core helical coil and disposed in the vicinity of the substrate to be polished, W.

[0047] The voltage detected between both ends of the sensor coil 41 passes through a band-pass filter 43 and then enters into a synchronous detecting section comprising a cos synchronous detector 45 and a sin synchronous detector 46. The cos synchronous detector 45 extracts a cos component of the detection signal from the voltage detected between both ends of the sensor coil 41. On the other hand, the sin synchronous detector 46 extracts a sin component of the detection signal from the voltage. An oscillation signal from an oscillating section 42 is formed into two signals, an in-phase component (0°) and an orthogonal component (90°), of a signal source by a phase shifting circuit 44. The signal of in-phase component (0°) is input to the cos synchronous detector 45 and the signal of the orthogonal component (90°) is input to the sin synchronous detector 46, and thus the synchronous detection as described above is carried out.

[0048] The signal that has experienced the synchronous detection is supplied to low-pass filters 47, 48 to remove any undesired high-frequency components containing higher frequencies than those of the signal components, so that ultimately the resistance component (R) output representing the cos synchronous detection output and a reactance component (X) output representing the sin synchronous detection output are extracted, respectively. If the substrate to be polished, W, intrudes between the lower surface of the sensor 8 and the top surface of the polishing table 1, the resistance component (R) output and the reactance component (X) output are changed, so that the escaping/slipping-off of the substrate to be polished, W, from the top ring 4 can be detected.

[0049] When the substrate to be polished, W, is made of insulating material, such as silicon wafer, the eddy-current type sensor could not have detected whether or not the substrate to be polished, W, is existing. In contrast to this, since the electrical resistance of the eddy-current type sensor 8 (i.e., the electrical resistance between the lower surface of the sensor and the polishing table 1) is measured and the slipping-off of the substrate to be polished is detected based on the change in resistance, any escaping/slipping-off of the substrate to be polished, W, can be detected in a short time (within 1 msec). When the escaping/slipping-off of the substrate to be polished, W, is detected, an appropriate action can be taken, including that the rotation of the top ring 4 and/or the polishing table 1 may be stopped, and/or that the top ring may be lifted up to separate the substrate to be polished, W, from the polishing surface.

[0050] Although in the above embodiment has been illustrated by way of example the case where the capacitance type or the eddy-current type sensors 8, 9 and 10 have been disposed in a plurality of locations (three locations in FIG. 1) surrounding the top ring 4, the present invention is not limited to this configuration. In an alternative embodiment, electrode(s) 17 of the sensor may be formed into a circular arc shape as shown in FIG. 7, wherein the electrode 17 may be disposed in the periphery of the top ring 4. This enables the substrate to be polished, W, if escaping/slipping-off from anywhere, to be detected without using a plurality of capacitance type or eddy-current type sensors.

[0051] In an abnormality detection signal of the substrate to be polished, such as a silicon wafer and so on, supplied by a current sensor or the like according to the related art, good stability and repeatability of the signal could not have been achieved without applying an averaging or a moving averaging processing to the signal. In case where the averaging or the moving averaging processing is applied, the possible delay time may reach 3.6 sec in maximum, resulting in too late detection of the abnormality to prevent the substrate to be polished, W, from slipping out of the top ring 4. In case where the eddy-current type sensor is employed as the sensor 8, by detecting which one of the resistance component (R) output or the reactance component (X) output has become greater than the normal level, as described above, it is possible to detect the escaping/slipping-off of the substrate to be polished, W, from the top ring 4. The necessary detection time in that case will be within 10 msec.

[0052] It is to be noted that although the above embodiment has been described on the case of the polishing table represented by the turntable, the polishing table is not limited to this, but it may be such a polishing table as shown in FIG. 8 comprising a polishing pad 16 in a form of endless belt bridging across a driven roller 14 and a driving roller 15, in which the rotation of the driving roller 15 may cause the polishing pad 16 to move. Alternatively, the polishing table may be a table of a scroll (translational circulation) movement type, which is not allowed to rotate around its own axis but is revolved with a small radius, or may be a table of controllable rotation and revolution type, in which a rotating speed and a revolving speed can be controlled independently from each other.

[0053] It is to be noted that although the above embodiments have been described on the example employing the capacitance type sensor or the eddy-current type sensor or the combination thereof as the sensor, it is a matter of course that the sensor may be configured with the capacitance type or the eddy-current type sensor in combination with an ultrasonic-type sensor for example, or may be configured with all of them in combination.

[0054] Although in the above embodiments, the polishing pad has been used as the polishing surface, a bonded abrasive (a grinding stone or a pad) composed of abrasive grains that have been bonded with binder may be employed. In that case, water (purified water) or chemical solution comprising a surfactant may be used. The chemical solution is used in order to appropriately adjust an autogenous amount of the abrasive grains and/or a polishing rate of the substrate may be supplied to the polishing surface as an abrasive liquid.

[0055] According to the above embodiments, owing to the configuration in which a set of sensor(s) comprising either one or both in combination of the capacitance type sensor that detects the escaping of the workpiece to be polished based on the change in capacitance between the sensor and the top surface of the polishing table and the eddy-current type sensor that detects the escaping of the workpiece to be polished based on the change in electrical resistance between the sensor and the top surface of the polishing table is disposed in one or more location(s) in the vicinity of the workpiece holding section, the escaping/slipping-off of the workpiece to be polished from the workpiece holding mechanism can be detected quickly (in a short time).

[0056] In addition, owing to the configuration in which the capacitance type sensor and/or the eddy-current type sensor are (is) mounted to the swing arm, the escaping of the workpiece to be polished can be detected quickly even during the swinging motion of the workpiece holding mechanism.

[0057] Further, owing to the configuration in which the capacitance type or the eddy-current type sensor comprising one ore more sensor electrode(s) having generally circular arc shape is disposed in the periphery of the vicinity of the workpiece holding section of the workpiece holding mechanism, the escaping of the workpiece to be polished from a broad extent can be detected quickly.

[0058] Although only some exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teaching and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.

[0059] The entire disclosure of Japanese Patent Application No. 2002-370858 filed on Dec. 20, 2002 including specification, claim, drawings and summery is incorporated herein by reference in its entirety. 

What is claimed is:
 1. A polishing apparatus comprising: a polishing table having a polishing surface; a workpiece holding mechanism for holding a workpiece to be polished, the workpiece held by said workpiece holding mechanism being pressed against said polishing surface of said polishing table so that said workpiece is polished through a relative motion of the workpiece to said polishing surface of said polishing table; and at least one of a capacitance type sensor, an eddy-current type sensor, and a combination of said capacitance type sensor with said eddy-current type sensor being disposed in at least one location in a vicinity of a workpiece holding section of said workpiece holding mechanism, said capacitance type sensor detecting an escaping of the workpiece to be polished based on a change in capacitance between said capacitance type sensor and a surface of said polishing table, said eddy-current type sensor detecting the escaping of the workpiece to be polished based on a change in electrical resistance between said eddy-current type sensor and the surface of said polishing table.
 2. A polishing apparatus in accordance with claim 1, wherein at least one of said capacitance type sensor and said eddy-current type sensor is mounted to a swing arm which supports and swings said workpiece holding mechanism.
 3. A polishing apparatus in accordance with claim 1, wherein at least one of said capacitance type sensor and said eddy-current type sensor comprises at least one sensor electrode having generally circular arc shape, and said sensor electrode is disposed in a periphery of the vicinity of said workpiece holding section of said workpiece holding mechanism.
 4. A polishing apparatus in accordance with claim 2, wherein at least one of said capacitance type sensor and said eddy-current type sensor comprises at least one sensor electrode having generally circular arc shape, and said sensor electrode is disposed in a periphery of the vicinity of said workpiece holding section of said workpiece holding mechanism.
 5. A polishing apparatus, comprising a polishing surface; a workpiece holding mechanism for holding a workpiece to be polished; a positioning apparatus for positioning said workpiece holding mechanism with respect to said polishing surface so that the workpiece to be polished, which is held by said workpiece holding mechanism, can be pressed against said polishing surface; a relative motion generating apparatus for generating a relative motion between said workpiece to be polished, which is held by said workpiece holding mechanism, and said polishing surface so as to thereby polish said workpiece to be polished; and a detecting apparatus for detecting escaping of said workpiece to be polished from said workpiece holding mechanism, said detecting apparatus including at least one of a capacitance type sensor and an eddy-current type sensor, said capacitance type sensor measuring a capacitance between said capacitance type sensor and said polishing surface; and said eddy-current type sensor measuring an electrical resistance between said eddy-current type sensor and said polishing surface; said detecting apparatus further including at least associated one of a first control section which detects the escaping of the workpiece to be polished from said workpiece holding mechanism based on a change in said capacitance supplied from said capacitance type sensor and a second control section which detects the escaping of the workpiece to be polished from said workpiece holding mechanism based on a change in said electrical resistance supplied from said eddy-current type sensor.
 6. A polishing apparatus in accordance with claim 5, wherein at least one of said capacitance type sensor and said eddy-current type sensor is disposed in at least one location in the vicinity of said workpiece holding mechanism.
 7. A polishing apparatus in accordance with claim 5, wherein said capacitance type sensor is disposed in one location in the vicinity of said workpiece holding mechanism; and said eddy-current type sensor is disposed in another location in the vicinity of said workpiece holding mechanism.
 8. A polishing apparatus in accordance with claim 5, wherein said positioning apparatus comprises a swing arm which supports and swings said workpiece holding mechanism; and said at least one of said capacitance type sensor and said eddy-current type sensor is mounted to said swing arm.
 9. A polishing apparatus in accordance with claim 5, wherein at least one of said capacitance type sensor and said eddy-current type sensor comprises at least one sensor electrode having generally circular arc shape; and said sensor electrode is disposed in a periphery of the vicinity of a workpiece holding section of said workpiece holding mechanism.
 10. A polishing apparatus in accordance with claim 5, further comprising a polishing cloth adapted to be removably attached to the polishing table, said polishing cloth including said polishing surface in a top surface thereof, wherein said capacitance type sensor measures a reference capacitance generated by a gap between a lower surface of said capacitance type sensor and the top surface of said polishing cloth and a thickness of said polishing cloth when said workpiece to be polished is held by said workpiece holding mechanism; said capacitance type sensor further measures a resultant capacitance generated by a gap between the lower surface of said capacitance type sensor and the top surface of said polishing cloth and the thickness of said polishing cloth when said workpiece to be polished is escaped from said workpiece holding mechanism and intruded into the gap between the lower surface of said capacitance type sensor and the top surface of said polishing cloth; and said first control section determines a difference between said reference capacitance and said resultant capacitance so as to detect the escaping of said workpiece to be polished from said workpiece holding mechanism.
 11. A polishing apparatus in accordance with claim 5, wherein said eddy-current type sensor measures a reference electrical resistance between a lower surface of said eddy-current type sensor and said polishing surface when said workpiece to be polished is held by said workpiece holding mechanism; said eddy-current type sensor further measures a resultant electrical resistance between the lower surface of said eddy-current type sensor and said polishing surface when said workpiece to be polished is escaped from said workpiece holding mechanism and intruded into the gap between the lower surface of said eddy-current type sensor and said polishing surface; and said second control section determines a difference between said reference electrical resistance and said resultant electrical resistance so as to detect the escaping of said workpiece to be polished from said workpiece holding mechanism.
 12. A polishing apparatus comprising: a polishing table having a polishing surface; a workpiece holding mechanism for holding a workpiece to be polished, the workpiece held by said workpiece holding mechanism being pressed against said polishing surface of said polishing table so that said workpiece is polished through a relative motion of the workpiece to said polishing surface of said polishing table; and an eddy-current type sensor being disposed in at least one location in a vicinity of a workpiece holding section of said workpiece holding mechanism, said eddy-current type sensor detecting the escaping of the workpiece to be polished based on a change in electrical resistance between said eddy-current type sensor and the surface of said polishing table. 